Tryptophan (Trp) is an essential amino acid necessary for the biosynthesis of proteins, niacin, and the neurotransmitter 5-hydroxytryptamine (serotonin). The heme-dependent oxygenase indoleamine 2,3-dioxygenase (also named INDO or IDO) is responsible for the extra-hepatic conversion of Trp to N-formyl-kynurenine as a rate-limiting first step of Trp metabolism. N-formyl-kynurenine is a precursor of a variety of bioactive molecules called kynurenines that have immunomodulatory properties (Schwarcz et al., Nat Rev Neurosci. 2012; 13(7):465-77).
IDO was initially described as part of the mammalian defense mechanism against parasite infection. Depletion of Trp can lead to growth arrest of intracellular pathogens such as Toxoplasma gondii or Chlamydia trachomatis (MacKenzie et al., Curr Drug Metab. 2007; 8(3):237-44). More recently, it has become apparent that IDO is an inducible enzyme that has a primary role in immune cell modulation. The reduction of Trp levels and increase in the pool of kynurenines cause inhibition of effector immune cells and promote adaptive immune suppression through induction and maintenance of regulatory T cells (Tregs; Munn, Front Biosci. 2012; 4:734-45).
Increased turnover of Trp to kynurenines by IDO has been observed in a number of disorders linked to activation of the immune system, e.g. infection, malignancy, autoimmune diseases, trauma and AIDS (Johnson and Munn, Immunol Invest 2012; 41(6-7): 765-97). Additional studies in these indications have shown that induction of IDO results in suppression of T-cell responses and promotion of tolerance. In cancer, for example, a large body of evidence suggests that IDO upregulation serves as a mechanism in tumour cells to escape immune surveillance. IDO is expressed widely in solid tumours (Uyttenhove et al., Nat Med. 2003; 10:1269-74) and has been observed in both primary and metastatic cancer cells. IDO is induced in tumours by proinflammatory cytokines, including type I and type II interferons that are produced by infiltrating lymphocytes (Tnani and Bayard, Biochim Biophys Acta. 1999; 1451(1):59-72; Mellor and Munn, Nat Rev Immunol 2004; 4(10):762-74; Munn, Front Biosci. 2012; 4:734-45) and TGF-Beta (Pallotta et al., Nat Immunol. 2011; 12(9):870-8). Certain oncogenic mutations can also lead to increased IDO expression, e.g., loss of the tumour suppressor Bin1 (Muller et al, Nat Med. 2005; 11(3):312-9) or activating mutations in KIT (Balachandran et al., Nat Med. 2011; 17(9): 1094-1100). IDO expression has been correlated with immune anergy in some tumours (e.g. Ino et al., Clin Cancer Res. 2008 Apr. 15; 14(8):2310-7; Brandacher et al., Clin. Cancer Res. 2006 Feb. 15; 12(4):1144-51), and a recent report has shown that reduction of IDO expression in human gastrointestinal tumours goes along with an increased infiltration of tumours by effector T cells (Balachandran et al., Nat Med. 2011; 17(9): 1094-1100).
A significant amount of preclinical data has been published that further validates the role of IDO in the anti-tumour immune response. For example, forced IDO induction in cancer cells was shown to confer a survival advantage (Uyttenhove et al., Nat Med. 2003; 10:1269-74). Other in vivo studies showed that IDO inhibitors cause lymphocyte dependent reduction in tumour growth by lowering kynurenine levels (Liu et al., Blood. 2010; 115(17):3520-30). Preclinical studies also highlighted the scope for IDO inhibitors to work synergistically in combination with agents that promote tumour antigenicity like irradiation, chemotherapy or vaccines (Koblish et al., Mol Cancer Ther. 2010; 9(2):489-98, Hou et al., Cancer Res. 2007; 67(2):792-801; Sharma et al., Blood. 2009; 113(24):6102-11).
In addition to creating an immune suppressive environment in tumours, IDO has also been implicated in inducing tolerance in lymph nodes, a phenomenon that seems to further contribute to immune evasion in cancer (Munn, Curr Opin Immunol. 2006; 18(2):220-5). IDO expression has been reported in antigen presenting cells, e.g. dendritic cells (DCs), which migrate to lymph nodes and induce anergy. IDO-positive DCs in tumour draining lymph nodes (TDLNs) of cancer-bearing mice have been shown to prevent the conversion of Tregs to inflammatory T-helper-17 (Th17)-like cells (Sharma et al., Blood. 2009; 113(24):6102-11), thereby blocking T-cell activation. Conversion of Tregs into proinflammatory Th17-like cells occurred when IDO activity was blocked with the IDO inhibitor 1-MT. IDO activity in TDLNs therefore provides an important aspect of the rationale for its inhibition as a cancer therapy.
IDO-mediated formation of kynurenines has recently also been implicated in mechanisms beyond the regulation of the immune system. For example, numerous studies since the 1970s have demonstrated that kynurenines can influence brain function. Kynurenine pathway metabolites are now seen as potential causative factors in several devastating brain diseases. Fluctuations in the level of kynurenine pathway metabolites can lead to the deterioration of physiological processes and the emergence of pathological states, e.g., neurodegenerative diseases, schizophrenia and depression (Schwarcz et al., Nat Rev Neurosci. 2012; 13(7):465-77). Furthermore, IDO-mediated kynurenine production in blood vessels has been linked to vasodilation and shock in inflammation and sepsis (Wang et al., Nat. Med 2010; 16(3):279-85). IDO expression has been observed in resistance vessels in human sepsis, and IDO activity correlates with hypotension in human septic shock (Changsirivathanathamrong et al., Crit Care Med. 2011; 39(12):2678-830). In clinical studies with sepsis and bacteremia patients, IDO-mediated tryptophan catabolism has been associated with dysregulated immune responses and impaired microvascular reactivity (Darcy et al., PLoS One. 2011; 6(6):e21185), as well as survival and disease severity (Huttunen et al., Shock. 2010; 33(2):149-54). Similarly, in community acquired pneumonia patients, IDO activity correlates with negative outcome and disease progression, including sepsis severity (Suzuki et al., J Infect. 2011; 63(3):215-22). There is therefore a strong rationale for inhibition of IDO activity in bacterial infections and sepsis.
Taken together, there is a need for the development of potent and selective IDO inhibitors, either as single agents or combination therapies, to modulate the kynurenine pathway and maintain physiological tryptophan levels in the body to more effectively combat diseases and conditions resulting from the harmful products of the kynurenine pathway, abnormal deviations in the levels of kynurenine pathway metabolites, or decreases in tryptophan levels. Such inhibitors counteract immune suppression, vasodilation and neurotoxicity that have been linked to the activity and expression of the IDO enzyme.